1. Field of the Invention
The present invention relates to amplifiers for amplifying signals having low magnitudes, and in particular, to transimpedance amplifiers used for amplifying signals having low magnitudes and containing DC offsets.
2. Description of the Related Art
Optical receivers responsible for receiving and amplifying data signals generated by optical signal systems typically use transimpedance amplifiers to receive and amplify the current based signal provided by a photodiode that receives the actual optical signal transmission. Such signals typically include a DC component, which often requires compensation so as to prevent biasing problems within the amplifier.
Referring to FIG. 1, a conventional transimpendance amplifier circuit for performing this function includes a gain stage in the form of an amplifier A1 (e.g., an operational amplifier) with a feedback resistor R2 between the input 3 and output 5 terminals. Also connected to the input terminal 3 is a current source I1 which provides a compensation circuit Ic (discussed in more detail below). The output signal voltage Vout also drives a lowpass filter circuit in the form of a serially connected resistance R1 and capacitance C1. The voltage V1 (which is the DC component of the output signal Vout) across the capacitor C1 is compared in a voltage comparison circuit A2 (e.g., voltage comparator) with a reference voltage Vref. The resulting signal V2 from this voltage comparison circuit A2 controls the current source I1, thereby controlling the compensation current Ic.
The photodiode D1 provides a signal current Is to the input node mutually connecting the current source I1, feedback resistor R2 and amplifier input terminal 3. When no light 1 is impinging upon the photodiode D1, the diode current Is is zero. During impingement of light 1 upon the photodiode D1, the diode current Is is non-zero and has a value determined by the responsivity of the photodiode and the input light power. If the incoming light signal 1 represents a data signal having a substantially equal number of ones and zeros, then the average photodiode current Isa will be equal to one-half of the peak photodiode current Is1 occurring during reception of a signal corresponding to a logical 1 (Isa=Is½). This average Isa current flows through the feedback resistor R2 of the transimpedance amplifier 4, thereby producing a potential difference between the DC bias points of the input terminal 3 and output terminal 5 of the amplifier A1. Accordingly, the DC bias point of the amplifier output terminal 5 is now dependent upon, i.e., varies with, the magnitude of the incoming signal, i.e., photodiode current Is, thereby making it difficult to maintain the DC biasing of the amplifier A1 for maximum dynamic range.
The conventional solution for this has been to provide the variable current source I1 at the input terminal 3 of the amplifier A1. The compensation current Ic provided by this current source I1 removes the DC component of the photodiode current Is. This is achieved by comparing the DC component V1 of the output signal voltage Vout (by lowpass filtering the output signal voltage Vout) with the reference Vref. If the DC output signal component V1 differs from the desired DC bias level, as represented by the reference voltage Vref, such voltage difference is caused by the DC component of the photodiode current Is flowing through the feedback resistor R2. Accordingly, the output signal V2 from the voltage comparison circuit A2 controls the current source I1 such that the compensation current Ic compensates for, i.e., removes, the DC component of the photodiode current Is at the amplifier input terminal 3, thereby causing the DC level at the amplifier output terminal 5 to return to and remain at the desired biasing point.
The conventional approach, however, is less than satisfactory for low values of photodiode current Is. As is well known, the current source I1 introduces thermal noise at the input node. The noise current resulting from such thermal noise is approximately equal to 4 αKTGm for metal oxide semiconductor devices (where α is a technology-dependent MOSFET parameter which is typically approximately ⅔, K is Boltzmann's constant, T is temperature in degrees Kelvin, Gm is the transconductance of the current source I1) and 2 QIc for bipolar junction transistors (where Q is the charge of an electron (1.6*10−19) and Ic is the compensation current). In low signal noise applications, i.e., where the photodiode current Is contains little signal noise, this thermal noise is the dominant noise source and degrades the signal-to-noise ratio (SNR), thereby causing an increased bit error rate (BER).